;; make sure t contains everything  -- this checks add.
;  el => (11 5 2 7 15 13 19)
;  t => (11 (5 (2 () ()) (7 () ())) (15 (13 () ()) (19 () ())))
;  (check-contents t el) => (#t #t #t #t #t #t #t)

;; remove 13 from t -- this checks delete on a leaf
;  t-13 => (11 (5 (2 () ()) (7 () ())) (15 () (19 () ())))
;  (check-contents t-13 el) => (#t #t #t #t #t 13 #t)

;; remove 13 from t-15 -- this checks delete on a node with only one child
;  t-13-15 => (11 (5 (2 () ()) (7 () ())) (19 () ()))
;  (check-contents t-13-15 el) => (#t #t #t #t 15 13 #t)

;; remove 5 from t -- this checks delete on a node with two children
;  t-5 => (11 (7 (2 () ()) ()) (15 (13 () ()) (19 () ())))
;  (check-contents t-5 el) => (#t 5 #t #t #t #t #t)

;; remove 11 from t -- this checks delete of root
;  t-11 => (13 (5 (2 () ()) (7 () ())) (15 () (19 () ())))
;  (check-contents t-11 el) => (11 #t #t #t #t #t #t)

;; check to see if t contains 99
;  (contains t 99) => ()